1. Field of the Invention
The present invention is directed to a method and to an apparatus for preventing mixing of different anaesthetics in an anaesthesia system.
2. Description of the Prior Art
During anaesthesia, the patient is usually supplied with a gas mixture containing nitrous oxide, oxygen and an anaesthetic. Generally, one of the anaesthetics desflurane, enflurane, isoflurane, halothane or sevoflurane is the anaesthetic employed. These are all liquid at room temperature (even though desflurane has a boiling point very close to normal room temperature).
The anaesthetic administered to the patient therefore is vaporized in an anaesthesia machine connected to the patient. Vaporization is usually performed by a vaporizer that can be devised in many ways. A number of different kinds of vaporizers are described in the literature and are well known to those skilled in the art. Since details of the vaporizer design and function are not of essential relevance to the present invention, they need not be specified below.
Since some liquid anaesthetic is consumed, however, occasions arise when the anaesthesia machine needs to be replenished with new anaesthetic.
Erroneous replenishment with an anaesthetic other than the anaesthetic currently in the anaesthesia machine is always a possibility, thereby posing a major risk to the patient.
Since the patient is at great risk during anaesthesia, there are a number of known procedures available for avoiding mixing.
One known procedure involves indexing. This means that the cylinders containing anaesthetics have different features. One known procedure entails assigning each anaesthetic a unique color. The contents of a color-coded cylinder may only be added to a correspondingly color-coded vaporizer in the anaesthesia machine.
Since this procedure is obviously not foolproof, systems employing this technique can be augmented with mechanical indexing, i.e. a cylinder of anaesthetic will only mate with a vaporizer equipped with a corresponding receptacle.
Even though this combination is safer than color coding alone, it is not completely safe. This is because different vaporizers around the world do not fit this indexing system because of their age or use of alternative technology. Vaporizers are sometimes emptied after use and the evacuated anaesthetic is re-used for inducing subsequent anaesthesia.
Yet another known procedure is to arrange a measurement unit inside the anaesthesia machine, either in the anaesthesia machine's gas flow paths or in an anaesthetic vessel in a vaporizer. The measurement unit is able to identify the anaesthetic, and anaesthesia can be stopped and/or an alarm sounded if the system detects any mixture of anaesthetics in the gas flow paths.
Since anaesthetics have similar molecular structures, they also have very similar properties. Therefore, reliable identification of even "pure" anaesthetic is difficult. Identifying mixtures of anaesthetics is even more difficult.